md_check_array_membership should pass export=True to mdadm_examine

Testing a first cut of subiquity with existing partitions failed to install even if I didn't touch the existing RAID I had set up. I think the underlying reason is this code in md_check_array_membership:

        dev_examine = mdadm_examine(device, export=False)
        if 'MD_UUID' not in dev_examine:
            raise ValueError('Device is not part of an array: ' + device)

mdadm_examine(device, export=False) is never going to return a dictionary with MD_UUID in it. Is it just supposed to be export=True?

I can't find any tests of type=raid, preserve=true so we'll see how many of these I find...